Bluetooth and WLAN share in common the 2.4 GHz ISM frequency band. When WLAN radio transmissions and Bluetooth radio transmissions overlap they can interfere with each other.
One solution to this problem has been adaptive frequency hopping (AFH) in Bluetooth. The Bluetooth transceivers do not use certain frequencies where there is interference because, for example, it is used by a WLAN transceiver. However, AFH requires frequency scanning which is not always practical and uses up radio resources.